The present invention relates to a process for operating the vapor phase epoxidation of ethylene in the presence of a supported highly selective silver based catalyst.
In the catalytic epoxidation of ethylene, modern silver-based supported catalysts are highly selective towards ethylene oxide production. Under certain operation conditions their selectivity towards ethylene oxide, expressed as a percentage of the ethylene converted, can reach values above the {fraction (6/7)} or 85.7 mol % limit which formerlyxe2x80x94based on the reaction formula 7 C2H4+6 O2xe2x86x926 C2H4O+2 CO2+2 H2O, see Kirk-Othmer""s Encyclopedia of Chemical Technology, 3rd ed. vol 9 (1980) p. 445xe2x80x94was considered to be the theoretically maximal selectivity of this reaction. Such highly selective catalysts, which may comprise as their active components silver, rhenium, at least one further metal and optionally a rhenium co-promoter, are disclosed in EP-B-266015 and in several subsequent patent publications.
Like all catalysts, the highly selective silver based ethylene epoxidation catalysts are subject to aging-related performance decline during normal operation and they need to be exchanged periodically. The aging manifests itself by a reduction in both selectivity and activity performance of the catalyst. Selectivity and activity are the primary (although not the only) determinants of plant profitability. There exists, therefore, a considerable economic incentive for delaying the need for exchanging the catalyst by preserving these values as long as possible. Several patent publications are known which are directed at stabilizing the catalyst by introducing modifications in the catalyst composition or in the support material, but thus far the reaction conditions and, in particular, the feed composition escaped attention in this respect.
It is known, for example from EP-A-567273, that when a fresh catalyst is used, operating at a higher concentration of ethylene and/or of oxygen in the reactor feed gas can lead to both a better activity and selectivity of the ethylene epoxidation reaction.
It has now surprisingly been found that aged ethylene oxidation catalysts react differently to the composition of the reactant gas mixture than do fresh ethylene oxidation catalysts, and that in this respect also highly selective catalysts differ from traditional catalysts. More specifically, when with the fresh highly selective catalysts the selectivity of the reaction towards ethylene oxide is not influenced substantially by employing a higher concentration of ethylene, with the aged highly selective catalysts the selectivity is substantially improved. Differences of activity performance under the same conditions of increased ethylene concentration between fresh and aged highly selective catalysts are in the same direction. By contrast to the highly selective catalysts, it has been found that aged and fresh traditional ethylene oxidation catalysts do not exhibit this difference in their reaction to the composition of the feed gas mixture.
The present invention therefore provides a process for the vapor phase oxidation of ethylene to ethylene oxide, which process comprises reacting a reaction mixture comprising ethylene and oxygen in the presence of a supported highly selective silver-based catalyst by:
operating at an initial operation phase wherein fresh catalyst is used, and
operating at a further operation phase when a cumulative ethylene oxide production exceeds 0.01 kT ethylene oxide per m3 of catalyst, wherein in said further operation phase the concentration of ethylene in the reaction mixture is increased.
In preferred embodiments, the invention provides a process for the vapor phase oxidation of ethylene to ethylene oxide in the presence of a supported highly selective silver-based catalyst, at a work rate w in the range of from 32 to 320 kg ethylene oxide produced per m3 of catalyst per hour, the reaction mixture containing ethylene, oxygen, optional carbon dioxide, gas phase moderator and balance inert gases, the reaction temperature being from 180 to 325xc2x0 C., the reactor inlet pressure from 1000 to 3500 kPa and the GHSV from 1500 to 10000, the process comprising:
operating at an initial operation phase wherein fresh catalyst is used, the reaction gas mixture containing an ethylene concentration which represents an economically optimized balance between catalyst performance (expressed, at the given work rate w, by the selectivity S in mol % and by the operating temperature T in xc2x0 C.) on the one hand and ethylene vent losses on the other, and an oxygen concentration which complies with safety-related flammability restrictions; and
operating at a further operation phase when the catalyst has reached an advanced aged defined by a cumulative ethylene oxide production exceeding 0.5 kT ethylene oxide per m3 of catalyst, in particular 1.5 kT ethylene oxide per m3 of catalyst, wherein in said further operation phase the composition of the reaction mixture is changed to contain from 1.1 to 4 times the concentration of ethylene used in the initial operation phase and the corresponding optimized and safe concentration of oxygen.
In further preferred embodiments, the invention provides a process for the vapor phase oxidation of ethylene to ethylene oxide in the presence of a supported highly selective silver-based catalyst, at a work rate w in the range of from 32 to 320 kg ethylene oxide produced per m3 of catalyst per hour, the reaction mixture containing ethylene, oxygen, optional carbon dioxide, gas phase moderator and balance inert gases, the reaction temperature being from 180 to 325xc2x0 C., the reactor inlet pressure from 1000 to 3500 kPa and the GHSV from 1500 to 10000, the process comprising:
operating at an initial operation phase wherein fresh catalyst is used, the reaction gas mixture containing an ethylene concentration which represents an economically optimized balance between catalyst performance (expressed, at the given work rate w, by the selectivity S in mol % and by the operating temperature T in xc2x0 C.) on the one hand and ethylene vent losses on the other, and an oxygen concentration which complies with safety-related flammability restrictions; and
operating at a further operation phase when the catalyst has aged sufficiently to cause the selectivity S to be reduced by at least 2.5 mol % and/or the activity parameter T to be raised by at least 15xc2x0 C., wherein the selectivity S and the activity parameter T are as defined hereinafter, and wherein in said further operation phase the composition of the reaction mixture is changed to contain from 1.1 to 4 times the concentration of ethylene used in the initial operation phase and the corresponding optimized and safe concentration of oxygen.